Whenever an optical character hand-held reader is held at an angle to the surface from which data is to be read, some tilt of the video field is produced. This tilt is caused mainly by uneven illumination resulting from unequal distances from the nose tip of the reader to the reading surface. The magnitude and direction of the tilt of the video is a function of the angular position of the hand-held unit. However, some readers have an inherent tilt bias due to gradient sensitivity characteristics is some arrays, small optical alignment errors or illuminator deficiencies.
These built in biases sometimes cause very marginal operation at the tilt angle limits that the reader is required to operate. Tilt in a vertical direction (top to bottom) is generally not a problem because of the ability of the AGC within the reader to change after each row has been scanned and thus track row to row changes. However, there is presently no compensation available in hand-held readers using an array for horizontal tilt. This causes the portion of the frame with lower video levels to threshold on minor noise signals, resulting in vertical black lines within a frame of video. The black lines often cause characters to be rejected.
The part of the problem which results from gradient sensitivity of the photodiode array has been kept within acceptable limits by maintaining a very tight limit in the manufacturing specification of the array and the testing and selecting of the arrays for use in hand-held readers. This has effectively resulted in an increase in cost of the sensor arrays because of the lowering of the yield.
The present invention provides a dynamic compensation for the horizontal tilt condition and insures a reasonable operational margin for the latitude tilt position and ultimately enables the specified tilt limits to be increased in the production specification of photodiode arrays.